I've noticed that in his books he recommends doing the aerobic running on the roads rather than on trails or grass. I know that he says running on the roads gives the best aerobic workout, whereas running on trails and grass gives a better muscular workout. But I would think that using the resistance of terrain to work the muscles harder would put just as high a level of stress on the cardiovascular system. Can anyone provide scientific support for Lydiard's reasoning?
Lydiard emphasized working on a specific system during certain parts of training, and the base phase was all about the cardiovascular system. Running on trails or grass provides more of a muscular workout, and the muscular system is worked on during hill phase, so cardiovascular work is all that is needed at that point. He just wanted to divide everything up into specific phases. Its probably not that important though.
(I believe) he liked the traction of roads vs. dirt, and the fact that marathons are on roads.
Wouldn't running on trails or grass also provide a good cardiovascular workout though (in addition to muscular)?
I think you also must look at the fact that very few (if any) smooth grass, dirt trails, or crushed asphalt existed back then like a good number of us have access to today. At the time he wrote his book the roads provided the best/smoothest surface to train on that was accessible to everyone. Personally, I like to keep a 50/50 grass to road ratio at any point in my training, keeps the injuries to a minimum.
A lot of the training his guys did was done at the end of the workday. It was often dark and often really wet. The roads were the best places to run. Also, as Trackhead mentioned, Arthur thought that because there was better traction on the roads than on trails or grass, you'd run at a faster speed with the same effort on roads than you would on dirt pr grass.
That said, lots of his original guys are fans of running on grass or dirt now. Arthur also had a beach near his house where he liked to send people for two hour runs. He showed it to me and said that he'd sent people like Coe and Viren out for two hour runs on that beach. He was also a big fan of Rotorua as a training ground and a lot of the runs there are in forests.